The present invention relates to gradation processing for color images in which gradation image data including a plurality of colors is recorded by a color printer by way of example, and more particularly to forming of an effective screen angle when a plurality of picture elements are used to represent one gradation.
The present invention also relates to an information converting method for use in converting color component gradation information read by such as a color scanner to color component recording gradation information applied to information recording equipment such as a color printer.
Certain digital recording apparatus can only record a unit recording picture element in a binary manner; "recording" or "non-recording". When using this type of digital recording apparatus, gradation cannot be represented for each unit of picture element, but it can effectively be represented by making a plurality of recording picture elements correspondent to one gradation information. A dither method or a density pattern method has been frequently utilized to represent such gradation.
When gradation is converted to binary data, there must be made judgment on "recording:1" or "non-recording:0" with a certain threshold set as the border level. Variation in this threshold permits representation gradation. In other words, by allocating different thresholds from each other in their values to respective elements of a two-dimensional matrix in X rows and Y columns and then comparing the thresholds of the respective elements with one or different gradation picture element data for conversion of gradation data to binary recording data, it is possible to change the number of picture elements above the "recording" level, i.e., recording density, in accordance with gradation of the gradation picture element data for each plurality of recording picture elements corresponding to the two-dimensional matrix of thresholds.
In the dither method, one threshold in the matrix is allocated with respect to the data for one gradation picture element, and location of the selected threshold is updated every time the matrix everywhen location of the gradation picture element is changed. In the density pattern method, all the elements of the matrix are made correspondent to the data of one gradation picture element and, when one gradation picture element data is input, the thresholds are selected in sequence to produce recording picture element information in accordance with the number of elementsof the matrix.
Meanwhile, when color recording is effected using ink of plural colors, such as Y (yellow), M (magenta) and C (cyan) for example, there may occur a moire due to interference between recording images of different colors. Also, because the threshold matrix utilized in converting gradation data to binary data is repeatedly used for each of the predetermined picture elements, the array pattern of thresholds may be remarked in the recording image.
Further, when recording locations of ink of different colors are shifted from each other due to mechanical positioning errors, such a shift appears in a certain direction throughout the entire image, and the resulting color shift will appear continuous, loud and marked image in the form of line, for example.
Moreover, depending on picture elements, ink of different colors may be recorded on a single picture element in superimposed relation. Theoretically, mixing of the three primary colors Y, M and C can reproduce any desired colors. In practice, however, depending on whether or not ink of different colors are superimposed, and the order of superimposition thereof, the reproduced color is changed and hence turbidity is caused in the reproduced color in relation to such factors as the transparency of the ink.
In the field of printing, the above-mentioned disadvantages in color recording have been heretofore solved by inclining a reticulated plate with respect to recording paper for each color by a predetermined angle, to such an extent that the adverse recording will not be practically appreciated by eyes of human beings. This inclination is generally known as a screen angle.
However, such a screen angle cannot be set in digital image recording of dot matrix system.
Accordingly, there has been proposed a method of practically forming a screen angle by causing an array of thresholds in the threshold matrix to have an inclination. But, the conventional method has the small degree of freedom in setting a screen angle, and a memory of very large capacity must be prepared for the threshold value in order to finely set the screen angle.
Meanwhile, color component information read by a color scanner consists of red (R), green (G) and blue (B) and, in case of gradation processing, the information is rendered to a signal level or digital data corresponding to color components of an image. For color recording on the basis of such color component information, because the main recording colors are yellow (Y), magenta (M) and cyan (C), the read color component gradation information R, G and B are converted to recording color component gradation information Y, M and C.
In case of color image reproduction due to color ink recording, for example, such information conversion has been heretofore performed following the equations below, which are known as masking equations, to correct a spectroscopic characteristic of the ink: EQU Y=a11R+a12G+a13B EQU M=a21R+a22G+a23B EQU C=a31R+a32G+a33B
To compensate failure of proportion and addition rules of ink, it can be also envisaged to perform color correction using such as the following masking equations including terms of higher orders: EQU i Y'a11R+a12G+a13B+a14RG+a15GB+a16BR+a17R.sup.2 +a18G.sup.2 +a19B.sup.2 EQU M'a21R+a22G+a23B+a24RG'a25GB+a26BR+a27R.sup.2 +a28G.sup.2 +a29B.sup.2 EQU C=a31R+a32G+a33B+a34RG+a35GB+a36BR+a37R.sup.2 +a38G.sup.2 +a39B.sup.2
In the past, arithmetic operation following the masking equations has been implemented as to the simple masking equations of first degree and conceived as to the complex masking equations of second or higher degrees with the aid of (a) an analog circuit using an operational amplifier, (b) a computer to conduct the desired calculation, (c) a digital arithmetic circuit in combination with a multiplier or an adder, or (d) a look-up table memory-digital system with which Y, M and C are read out through accessing with R, G and B as parameters. The method of (a) has a high processing speed, but requires a number of operational amplifiers to implement the complex masking equations, thus resulting in a very complicated circuit configuration. The method of (b) employs a microprocessor to calculate the complex masking equations and hence a low processing speed. The method of (c) requires a number of expensive multipliers and hence must resort to a very complicated circuit configuration. Finally, with the method of (d), if 8 bits are allocated to each of R, G and B by way of example, there are needed memory addresses of 2.sup.24 and the memory capacity of 2.sup.24 (abut 16M) words, thus resulting in the enormous memory capacity to be required. As mentioned above, for conventional color information conversion, it is difficult to perform the masking equations for second or higher degrees.